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How to extend the service life of reverse osmosis membrane?
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How to extend the service life of reverse osmosis membrane?

Views: 83     Author: Site Editor     Publish Time: 2021-03-22      Origin: Site

Reverse osmosis membrane is an important part of water treatment equipment , widely used in electronics, electric power, automotive, food, medical and other fields. All mechanical equipment has a service life, and the service life of water treatment equipment has a lot to do with the life of the reverse osmosis membrane, in addition to design and material. Here, Amanda shares with you the methods and tips on how to maintain the reverse osmosis membrane and extend the service life of reverse osmosis water treatment equipment.

After the commissioning of reverse osmosis equipment, two methods are used to protect the membrane. Equipment test run for two days (15 ~ 24h), then use 2% formaldehyde solution maintenance; or run 2 ~ 6h after the maintenance with 1% of the aqueous solution of NaHSO3 (should exhaust the air in the equipment pipeline, to ensure that the equipment does not leak, close all the import and export valves). Both methods can get satisfactory results.

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Reverse osmosis equipment has residual gas operating at high pressure, the formation of gas hammer will damage the membrane often occurs in two ways: after the equipment is emptied, re-run, the gas is not exhausted on the rapid ramp-up operation. The remaining air should be exhausted at a pressure of 2 to 4 bar and then gradually ramped up. When the joint between the pretreatment equipment and the high-pressure pump is not sealed well or leaked (especially the microfilter and the pipeline after it is leaked), when the water supply of the reverse osmosis pretreatment is not sufficient, the microfilter is clogged and some air will be sucked in due to the vacuum in the place where it is not sealed well. The microfilter should be cleaned or replaced to ensure that the pipeline does not leak. The pressure should be gradually increased to run without bubbles in the flow meter, and the bubbles found in the operation should be gradually decreased to check the reason.

As the concentration of inorganic salts on the concentrated water side of the membrane is higher than that of the raw water, it is easy to scale and pollute the membrane. Flushing with pretreatment water with chemical reagents. Water containing chemical reagents may cause membrane contamination during equipment shutdown. When the reverse osmosis equipment is ready to shut down, it should stop adding chemical reagents and gradually lower the pressure to about 3 bar and flush with pretreatment water for 10 min until the TDS of concentrated water is very close to that of the original water.  

The common problem in the use of composite polyamide membrane, because of the poor residual chlorine resistance of polyamide membrane, in use without the correct injection of chlorine and other disinfectants, coupled with the user does not pay enough attention to the prevention of microorganisms, easily lead to microbial contamination. At present, many water treatment equipment manufacturers produce pure water microbial exceedance, is the disinfection, maintenance is not effective caused by. Reverse osmosis equipment does not use disinfection solution maintenance; equipment installed without the entire pipeline and pretreatment equipment disinfection; intermittent operation does not take disinfection and maintenance measures; no regular disinfection of pretreatment equipment and reverse osmosis equipment; maintenance solution failure or insufficient concentration.

In the process of using reverse osmosis equipment, in addition to the normal decay of performance, the decay of equipment performance due to pollution is more serious. Usual pollution mainly includes chemical scale, organic and colloidal contamination, microbial contamination, etc. Different pollution symptoms are different.

① Colloid contamination: When colloid contamination occurs, it is usually accompanied by the following two characteristics: the microfilter in the pre-treatment is clogged quickly, especially the differential pressure increases quickly, and the B, SDI value is usually above 2.5.

②Microbial contamination: When microbial contamination occurs, the total number of bacteria in the permeate water and concentrated water of the reverse osmosis equipment is relatively high, and usually must not be maintained and disinfected as required.

③Calcium scale: It can be judged according to the raw water quality and design parameters.

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Reverse osmosis equipment pretreatment debugging

      The quality of pre-treatment is the key to the stable operation of reverse osmosis equipment. When the feed water of reverse osmosis equipment adopts groundwater, the pretreatment of quartz sand and activated carbon is not a problem, but it is different when using surface water.

    1、Reverse osmosis equipment pretreatment chemicals

      With the agents used in the treatment, including coagulants, coagulants, oxidants, reducing agents, scale inhibitors, etc., especially coagulants and scale inhibitors, the selection of these agents, the dosing amount and even the method of drug distribution will have a great impact on the operation of reverse osmosis.

      Usually, we will pay attention to detect the iron content of the feed water, but in fact, the high aluminum content in the feed water can also cause the contamination of the reverse osmosis membrane. Aluminum contamination of membrane is caused by aluminum hydroxide precipitation, which usually exists in colloidal form and is an amphoteric hydroxide with small solubility in the pH range of 6.5-6.7. If the aluminum coalescence process is carried out at too high or too low pH, aluminum ions will enter the reverse osmosis unit and cause contamination of reverse osmosis membrane, so for the pretreatment system using aluminum salt as coagulant, the pH value should be controlled at 6.5-6.7. system, the pH value is controlled at 6.5-6.7 to make the solubility of aluminum reach small, pay attention to adjust the dosing amount timely according to the water quality, and if possible, the aluminum content in the pretreated water should be tested regularly and controlled below 0.05mg/L.

      In order to prevent the scaling of the concentrated water side, we usually add scale inhibitor, the current scale inhibitor to some organic acids, organic phosphate substances compounded to achieve the purpose of scale inhibition, dispersion. If not selected or controlled properly, these organic substances will foul the reverse osmosis membrane element, and at the same time it will become a breeding ground for bacterial microorganisms and bring more harm to the operation of reverse osmosis.



      As you may know, temperature has a great impact on the flux of the reverse osmosis membrane element, so the temperature is calibrated when calculating the water yield in order to compare with each other. Therefore, in winter, where the water temperature is low, the pre-treatment system of reverse osmosis will be designed with heating equipment, which can effectively ensure that the reverse osmosis equipment can also reach the designed output in winter.

      In fact, SiO₂ precipitation in the membrane element is also closely related to the temperature of the feed water of the reverse osmosis device, and the concentration of silica in the concentrated water cannot exceed 100mg/l at 25℃ and 25mg/l at 5℃, therefore, when there is no heating equipment in the pretreatment system, we should pay great attention to the pollution of the membrane element by silica precipitation in winter, and strictly control the content of silica in the concentrated water. The value should not exceed the solubility at that temperature.

Operation management of reverse osmosis equipment

    1、Regular inspection of reverse osmosis equipment

    Regular inspection and timely replacement of the security filter cartridge to prevent particle contamination of the reverse osmosis membrane caused by the leakage of the cartridge due to installation or quality problems. When the differential pressure of the security filter inlet is greater than 0.15MPa, the cartridge should be replaced. Generally, it should be checked once a month, and the cartridge should not be used for more than 6 months, and the security filter should also be checked frequently when it is running is enough to have gas inside, so that air cannot be brought in.

    2、Inspection of reverse osmosis membrane element

   (1) Generally every six months (can be shortened if necessary), each set of reverse osmosis of a section, two sections of membrane elements to check.

    (2) open the end cover of the pressure vessel (using special tools and operated by skilled mechanics).

    (3) Check whether there are mechanical impurities and metal oxides deposited in the inlet section, whether there is bacterial and microbial growth, whether there is any change in the color of the membrane element, and whether there is membrane fouling.

    (4) If necessary, you can also pull out the reverse osmosis membrane element for detailed inspection, and then pump the inlet end of the membrane element, must not be directly pumped, to put the direction of water flow it from the pressure vessel to push out, the same when installed.

    (5) After each inspection to make detailed records for comparison.

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    3、The meters are regularly calibrated to ensure that the instruments are accurate and reliable.

    4、Regularly analyze the statistics of reverse osmosis equipment operation data.

      Operating pressure, recovery rate (or concentrated water discharge), SDI (pollution index) of feed water, pH, residual chlorine and temperature are the main operating control parameters of the reverse osmosis plant; desalination rate, water production and differential pressure are the three main monitoring performance parameters. They must be strictly observed in operation and management, and the operating conditions must not be changed at will. In particular, it is necessary to prevent increasing the recovery rate to increase the water production, resulting in scaling of the reverse osmosis membrane surface; to prevent continuing operation with SDI value exceeding the standard, resulting in clogging of the reverse osmosis membrane; and to prevent destructive damage of the membrane element by continuing work above the allowed large differential pressure.